This invention relates to a method of fabricating a microbridge structure for a micromachined device. It is particularly, but not exclusively, related to fabricating a microbridge for a radiation detector element.
The use of the pyroelectric effect in thermal detectors is well established. In one embodiment a known thermal detector has a 2-dimensional array of thermal detector elements coupled to a readout circuit to give a staring imaging device. The detector elements are comprised of ferroelectric material. Various technologies have been developed to manufacture such detectors. These include hybrid technologies where the readout circuit and ferroelectric element array are fabricated separately and joined in a bonding process such as flip-chip bump bonding. Alternatively integrated technologies are used in which the array is fabricated from a thin film of ferroelectric material deposited directly onto the readout circuit. The detector elements are then defined by micromachining processes.
A requirement for thermal detector elements is thermal isolation from the substrate, which can be achieved by forming microbridge structures separated from the substrate except for very small links. Separation from the substrate is achieved by fabricating the microbridge structure over an etchable sacrificial layer which is removed after the microbridge has been formed.
The ferroelectric materials may be used below their Curie temperature after poling (for pyroelectric devices) or above the Curie temperature with an applied electric DC bias field (for dielectric bolometer devices).
Some processes for deposition of ferroelectric thin films (for example deposition from solution) are planarising processes, where the growing film tends to be smoother than the substrate surface upon which it is grown. It is often the case that the circuit on which the ferroelectric thin film is deposited, for example a readout circuit, is not planar but is uneven having circuit features such as interconnecting tracks and vias. If a ferroelectric film is deposited on such an uneven surface, this can cause problems with the performance of thermal detector elements because the film thickness varies with position on the circuit. As a consequence it is not possible to achieve a uniform electric field across all of the ferroelectric film during poling (for pyroelectric devices) or in operation (for dielectric bolometer devices).
The electric field applied to the film is dependent upon the film thickness. For very thin regions the voltage applied across the film has to be limited to avoid breakdown of the film. In this event a reduced electric field is applied over thicker regions of the film. This will lead to loss of radiometric performance and an increased tendency to break down under an applied field either during poling or in operation.
One method of tackling the problem of uneven film thickness is to micromachine microbridges on planar areas of the substrate which are not covered by surface features such as tracks and vias. However this reduces the fill factor on the substrate and leads to the ferroelectric array having a sparse distribution. This is inefficient in using up the space on the substrate which is disadvantageous because a higher fill factor produces better imaging.
It is an aim of the invention to overcome at least some of the problems set out above.